1. Field of the Invention
The invention relates to an individual rechargeable electric cell with liquid sodium as the negative and liquid sulfur, absorbed in graphite, as the positive electrochemical material as well as with a sodium ion-conducting solid electrolyte which is disposed in the metallic housing of the individual cell as a container, which is open on one side and contains the sulfur and the sodium, respectively, and includes a current collector.
2. Description of the Prior Art
It is known that an individual rechargeable electric cell may consist substantially of melted sodium as one reaction substance and melted sulfur as the other, with the two reaction partners separated by a solid-electrolyte tube. At the operating temperature of the Na/S cell of 300.degree. to 350.degree. C., this tube has a high sodium-ion conductivity. Sulfur is absorbed in an electron-conduction graphite felt. The latter with its large surface forms the extension of the metal housing serving as the positive terminal and that aids in assuring that the electrochemical processes proceed fast enough. The metallic lid which is in contact with the sodium via a metal pin serves as the negative terminal. During discharge, when both terminals are connected to each other via a load, sodium migrates as an ion to the sulfur through the electrolyte tube. In the process, the sodium ions react with sulfur to form sodium polysulfide, absorbing electrons. In the course of the discharge, more and more sodium is therefore used up from the interior, and the sodium level drops. At the same time, more and more sodium polysulfide is formed, and the level in the outer space rises. The fully discharged cell contains Na.sub.2 S.sub.3 as the reaction product in the outer space.
During the recharging of the cell, all processes are reversed. The reaction product Na.sub.2 S.sub.3 is decomposed, the sodium ions migrate through the solid electrolyte back into the interior, and sulfur remains in the outer space. The operating temperature of the Na/S cell is above the melting points of sodium, sulfur and the sodium polysulfides, and therefore, the reactants and the reaction products are liquid. In this manner the contact necessary for the electrochemical process to proceed between the solid electrolyte and the reactants can be maintained continuously. To maintain the operating temperature, which is substantially above the ambient temperature, the cells are surrounded, as is well known, by an insulating housing.
One of the most important components of an Na/S cell is the solid electrolyte. The suitability of .beta.-aluminum oxide (.beta.-Al.sub.2 O.sub.3) as the electrolyte is based on its high conductivity and its long life. In such an individual cell, the sealing of the sodium and the sulfur space can be accomplished in such a manner that the tubular electrolyte has at its open end a shoulder, via which a rigid connection to the metallic housing of the individual cell can be made. The rigid connection is usually made by means of a plate consisting of alloy steel and rings of aluminum can be used as the seal between the alloy steel and electrolyte. The alloy steel plate in turn can be connected firmly to the housing. This design principle has the consequence that the ceramic electrolyte must take up, due to the rigid connection to the housing, any pressure differences which can occur especially due to temperature differences or by a change in the charging state. Since ceramics are brittle by nature, such an arrangement can easily lead to a fracture of the ceramic (electrolyte) and thereby to a failure of the entire cell.